The present invention relates generally to forming three-dimensional parts from a generally flat sheet or panel of metal, and particularly to a process in which the formability of the sheet or panel is substantially increased and controlled.
In conventional stretch or wrap forming, a sheet of metal is clamped by jaws, and a tool surface located between the jaws is moved into the sheet, with the jaws free to rotate to stretch the sheet over the tool surface. In stretch wrap forming, jaws are actuated such that the sheet can be stretched by displacement of the jaws either prior to or during motion of the center tool. A second set of jaws normal to the first set can be used to stretch the sheet in two directions over the center die surface.
The jaw motion during stretch wrap forming required to produce a desired part is generally determined by trial and error during a set-up of the stretchwrap apparatus to avoid wrinkling or tearing of the part while producing the desired shape after unloading. Material properties and initial thickness variability leads to variability in shape of the component produced (after spring-back) and to possible fracturing or wrinkling of the metal part. The motion of the double jaw sets located essentially normal to each other can be specified to result in strain paths which cause a forming limit curve or diagram to shift, resulting in improved formability along certain specific strain paths.
The study of formability received a substantial boost in the mid-1960's when the forming limit diagram was introduced, as discussed in the following list of papers:
S. P. Keeler, "Determination of Forming Limits in Automotive Stampings", Sheet Metal Industries, 42(461), 1965, pp. 683-691 PA0 S. P. Keeler, "Circular Grid System A Valuable Aid for Evaluating Sheet Metal Formability", SAE Paper No. 680092, 1968 PA0 G. M. Goodwin, "Application of Strain Analysis to Sheet Forming Problems in the Press Shop", SAE Paper No. 680093, 1968 PA0 K. Nakazima, T. Kikuma and K. Hisuka, Yamata Technical Report No. 264, 1972, p. 141 PA0 R. Pearce, Sheet Metal Forming, Adam Hilger, Bristol, 1991, pp. 143-175 PA0 S. Dinda, K. F. James, S. P. Keeler and P. Stine, How to Use Grid Circle Analysis for Die Tryout, ASM, 1981. PA0 D. D. Olander and A. K. Miller, "The Influence of Constitutive Behavior on Predicted Sheet Metal Forming Limits", Controlling Sheet Metal Forming Processes, Proc. 15th Biennial Congress IDDRG, 1988, pp. 133-144; PA0 A. Barata Da Rocha and J. M. Jalinier, "Plastic Instability of Sheet Metals Under Simple and Complex Strain Paths", Transactions of the Iron and Steel Institute of Japan, 24, 1984, pp. 132-140; PA0 F. Barlat, Barata Da Rocha and J. M. Jalinier, "Influence of Damage on the Plastic Instability of Sheet Metals Under Complex Strain Paths", J. Materials Science, 19, 1984, pp. 4133-4137; PA0 A. Barata Da Rocha, F. Barlat and J. M. Jalinier, "Prediction of the Forming Limit Diagrams of Anisotropic Sheets in Linear and Nonlinear Loading", Materials Science and Engineering, 1984, pp. 151-164. PA0 A. Graf and W. F. Hosford, "Effect of Changing Strain Paths on Forming Limit Diagrams of A1 2008-T4", Metallurgical Transactions, Vol. 24, No. 11 (November 1993), pp. 2503-2512.
The forming limit diagram or curve is a map of the combinations of surface strains leading to success/failure in a sheet stamping operation. Forming limit diagrams are usually generated by stretching gridded samples of various widths over a hemisphere punch and into a die cavity under various conditions of friction. Major and minor strains are defined as the larger and smaller of the two in-plane principal strains, respectively. Circular grids are often used because they deform into ellipses, the major and minor axes of which can easily be used to calculate the principal strains. Wide samples with very good lubrication result in positive minor strains only slightly smaller than the major strains. Reducing sample width mid/or increasing friction results in smaller minor strains. There will be a sample width and friction condition which results in zero minor strain. Narrower samples yield negative minor strain. In FIGS. 1 and 6, major strain is the ordinate and minor strain is the abscissa.